1. Field of the Invention
The present invention relates to a differential amplifier using body-source cross coupling, and more particularly, to a differential amplifier using body-source cross coupling, which is capable of increasing transconductance due to body effect to increase a gain and suppressing a breakdown voltage from being reduced, by cross coupling bodies and sources of common gate amplifiers in a common gate differential amplifier in which the common gate amplifiers are implemented in a differential structure.
2. Description of the Related Art
FIGS. 1 and 2 are circuit diagrams showing a conventional common gate differential amplifier using NMOSFETs and PMOSFETs, respectively.
As shown, in a differential structure, common gates 9 and 29 form virtual grounds and a DC voltage is applied to the gates to form a bias circuit. Current sources 3, 4, 23 and 24 are provided to source terminals 5, 6, 25 and 26 of MOSFETs so as to maintain high impedance while operating current flows. Loads 10, 11, 30 and 31 are provided to drain terminals 7, 8, 27 and 28 of the MOSFETs. The bodies of the MOSFETs are tied to the source of the respective MOSFETs to remove body effect. Differential signals are input to the source terminals 5, 6, 25 and 26 of the MOSFETs, which are differential input terminals, and amplified differential signals are output from the drain terminals 7, 8, 27 and 28 of the MOSFETs, which are differential output terminals.
FIG. 3 is a circuit diagram showing a conventional cascode differential amplifier using NMOSFETs. DC voltages are applied to gates 305 and 306 of common source amplifiers 303 and 304 and a common gate 309 of common gate amplifiers 301 and 302 to form a bias circuit. The common gate of the common gate amplifier is differentially operated in a virtual ground state. The bodies and the sources of the common source amplifiers 303 and 304 and the common gate amplifiers 301 and 302 are respectively tied so as to remove body effect. Differential signals are input to the gates 305 and 306 of the common source amplifiers 303 and 304, which are differential input terminals, and amplified differential signals are output from the drain terminals 307 and 308 of the common gate amplifiers 301 and 302, which are differential output terminals.
In such a differential amplifier, the body effect improves a gain of the common gate amplifier, but reduces a breakdown voltage of the transistor, because the potential of the body is lower than that of the source.